DISTRIBUTION AND INTERPRETATION OF STRATA 353 



Ordovician and the New York Clinton of the Silurian to which the name 

 Alexandrian series has been applied. 



Distribution of Strata 



The rocks representing the Alexandrian series in Illinois and eastern 

 Missouri appear at the surface in two general areas that are separated by 

 a distance of 200 miles. One area borders both sides of the Mississippi 

 Eiver from about the mouth of the Ohio Eiver north nearly to Hannibal, 

 Missouri. Within this belt outcrops occur at intervals over a width of 

 25 to 40 miles. Good exposures have been studied in Alexander and 

 Union counties, Illinois, and in the vicinity of Cape Girardeau, Missouri. 

 North of the fault-line that crosses the south end of Jersey and Calhoun 

 counties, Illinois, and west into Lincoln County, Missouri, strata repre- 

 senting some part of this series outcrop at a number of places over the 

 east part of Lincoln, Pike, and Balls counties, Missouri, and in Jersey, 

 Calhoun, and Pike counties, Illinois, within a distance of 12 to 20 miles 

 from the Mississippi Eiver. 



The second area in which these early Silurian rocks are exposed in the 

 upper Mississippi Valley is in Will and Kankakee counties, in north- 

 eastern Illinois, where somewhat isolated outcrops of the Sexton Creek 

 (=Brassfield) limestone and earlier Silurian strata appear at a number 

 of places in the vicinity of Kankakee and Desplaines rivers. It is prob- 

 able that the Alexandrian strata in the Mississippi Eiver area and in 

 northeast Illinois are more or less continuous, probably along the present 

 basin of the Illinois Eiver, but they are concealed beneath younger de- 

 posits in the intervening region. 



Earlier Studies and Interpretation of the Strata 



In a report in 1855, Mr. G. C. Swallow, 5 who was then State geologist 

 of Missouri, noted the white oolite bed in the vicinity of Louisiana, Mis- 

 souri, and referred it to the Onondaga of the Devonian. 



In the same report Shumard 6 described a limestone occurring near 

 Cape Girardeau, Missouri, under the name Cape Girardeau limestone, 

 and considered it the oldest Silurian formation in the State. 



In discussing the stratigraphic geology of Illinois in 1866, Worthen 7 

 recognized the Girardeau limestone in the vicinity of Thebes, but referred 

 it to the Ordovician in the upper part of the Cincinnati group. 



5 G. C. Swallow: Geological Survey of Missouri. Reports 1 and 2, 1855, p. 107. 

 6 B. F. Shumard: Geological Survey of Missouri. Report 2, 1855, p. 154, 

 7 A. H. Worthen; Geological Survey of Illinois, vol. i, 1866, p, L30, 



